An exemplary automated manufacturing line may include several hundred electrically powered devices arranged in device sets at separate manufacturing stations, a separate manufacturing process performed by each device at each station. For example, the devices may include robots, drills, mills, transfer lines, clamps, mixing machines, stuffing machines, drying machines and so on, each of which is linked to one or more loads such as motors for driving the devices through required movements and processes.
When designing a manufacturing or processing line one of the primary considerations is line safety as many of the devices at each line station may inflict injury to an operator in the station vicinity. Typical injuries including mechanical injury (e.g., falling, crushing, puncture, etc.). For this reason many stations, and in some cases all stations, will be enclosed in a housing assembly to ensure that an operator does not inadvertently enter a potentially hazardous station environment. Hereinafter an exemplary enclosure will be referred to as a station and the station or device grouping therein will sometimes be referred to as a hazard to indicate the potential danger associated therewith.
Despite painstaking design of the processing line stations and of the control method associated therewith, often processing problems can occur which require operator intervention to alleviate the problems. To facilitate operator intervention, typically line access points are provided. To ensure that an operator entering a station via an access point is not injured, standard practice within the industry requires that power to the station be entirely cut off via a power down mechanism. By cutting off power to the station, all station devices cease mechanical movement and the possibility of injury is essentially eliminated.
In copending U.S. patent application Ser. No. 09/407,475, of which the present patent application is a continuation-in-part, and which is incorporated by reference herein, a new system for disconnecting manufacturing stations (and their respective devices/loads) from power is described. The system, which is again described below with reference to FIG. 2, operates to disconnect manufacturing stations from power in two ways. First, the manufacturing stations are electrically isolated (e.g., disconnected) from the power source itself. Second, the manufacturing stations are then connected to ground. The coupling of the manufacturing stations to ground ensures that no power is provided to those stations.
While this system for electrically isolating and grounding manufacturing stations is an advance over other systems, the grounding of the manufacturing stations can have deleterious effects under certain operational circumstances. For example, certain devices/loads such as variable frequency drives can experience regenerative braking upon being disconnected from the power source. If the terminals on the variable frequency drives are immediately coupled to ground upon being disconnected from power, the sudden changes in voltages and currents experienced at the terminals and within the drives can cause damage to the drives. Consequently, it would be advantageous if a new system for electrically isolating and grounding manufacturing stations could be developed that avoided causing the deleterious effects to manufacturing station loads that otherwise can occur upon the grounding of those loads.